The application of decorative linings on a previously shaped substrate is a common and necessary action for providing the decorative finish to said substrate shaped to define a headliner. Consequently, the decorative lining has a mainly aesthetic function.
To fulfil said aesthetic function it is necessary to ensure proper adhesion throughout the entire contact surface between the decorative lining and the shaped substrate, while preventing the decorative lining from getting dirty during the joining process.
The conventional way to prevent the decorative lining from getting dirty is to reduce the exposure thereof to the adhesive by applying said adhesive on the shaped substrate instead of on said decorative lining. One example in which this solution is shown is patent ES-8400291.
Since the shaped substrate has a three-dimensional shape, normally with complex shapes, it is necessary to use adhesive application methods that allow the adhesive to reach all of the surface that will be in contact with the decorative lining.
The most common method comprises applying the adhesive by spraying, which requires using liquid adhesives with low viscosity by adding a solvent to the adhesive, normally water or an organic solvent.
Despite the low viscosity of the liquid adhesive, the application on a shaped substrate with low porosity allows an optimum use thereof, which is not the case when applied on a decorative lining which, due to its porosity, absorbs part of the adhesive reducing the effectiveness thereof, and potentially even leading to a transfer of the adhesive with the resulting soiling of the decorative lining.
One of the drawbacks of spraying the adhesive on the three-dimensional shaped substrate is achieving a sufficient distribution of the adhesive to obtain good adherence, which also lasts for the useful lifetime of the headliner. This requires applying excess adhesive. Specifically, the resulting adhesive layer, after the adhesive has set, in the headliner obtained has a grammage of 60 to 80 g/m2.
Another drawback of the use of liquid adhesives is the need to remove the solvent once the adhesive has been applied to the shaped substrate.
Specifically, in the case of thermosetting liquid adhesives comprising volatile organic compounds as solvent, it is necessary to provide an additional installation for capturing and treating said volatile organic compounds, increasing the cost and complexity of use of this type of thermosetting liquid adhesives.
In the case of thermosetting liquid adhesives in which the solvent is water, although this avoids the problem of emission and treatment of volatile organic compounds, it has be drawback of requiring a drying chamber for eliminating the water contained in the adhesive after application on the shaped substrate.
In view of the above, the subject matter of the invention consists in a process for manufacturing vehicle headliners, and specifically a process for joining a decorative lining to a shaped substrate by the use of a thermosetting adhesive, wherein a control is established of the adhesive application by a sufficient distribution thereof, resulting in proper adhesion throughout the entire contact surface between the two components that is lasting and prevents soiling the decorative lining, such that the achievement of the said objectives does not increase the cost of the installation or the materials used to perform the adhesion between the decorative lining and the shaped substrate.